1. Field of the Invention
The present invention relates to a method of suppressing deposition of radionuclides on components of a power plant such as a nuclear power plant, and a ferrite film formation apparatus for performing the radionuclide deposition suppression method.
2. Background Arts
For example, in a nuclear power plant of boiling water type reactor (hereunder abbreviated as BWR), the heat generated by fuel is effectively transferred to cooling water by forcibly circulating the cooling water by a primary loop recirculation pump and internal pumps inside the nuclear reactor comprising a reactor pressure vessel and control rods contained therein. Most of the steam generated from the cooling water in this way is used for driving a steam turbine generator, the steam exhausted from the steam turbine is condensed in a condenser, and the condensate condensed in the condenser is almost fully deaerated and supplied again as cooling water of the nuclear reactor. In this case, in the condenser, oxygen and hydrogen generated by decomposition of water by radiation are also almost completely removed. Further, the condensate to be returned to the nuclear reactor is supplied to the nuclear reactor after removing mainly metal impurities by an ion-exchange resin filter and being heated to nearly 200° C., in order to suppress occurrence of activated corrosive products in the nuclear reactor.
Further, since the activated corrosive products are generated inside the rector pressure vessel and at water-contact portions of a recirculation system, stainless steel such as nickel-base metal which is less in corrosion is used for components of the main primary system. Further, for a reactor pressure vessel made of low alloy steel, inside building up of stainless steel is performed, thereby to prevent the low alloy steel from contacting directly with the reactor water. In addition to taking into a consideration the above-mentioned matters, a part of the reactor water is purified by a reactor-water cleanup facility and metal impurities produced slightly in the reactor water are positively removed.
However, even if the above-mentioned measures for corrosion prevention are taken, very slight metal impurities exist inevitably in the reactor water, so that a part of the metal impurities deposits on the surface of a fuel rod as metal oxide. Metal elements deposited on the surface of the fuel rod are irradiated by neutrons radiated from the fuel to cause nuclear reaction, whereby radionuclides such as cobalt 60, cobalt 58, chromium 51, manganese 54, etc are produced. Most of those radionuclides are, as they are, deposited on the fuel rod surface in a state of oxide, however, a part of the radionuclides is solved out into the cooling water according to the solubility of oxides taken in and released again into the reactor water as insoluble solid called cruds. Radioactive substances in the reactor water are removed by a reactor-water purifying system, however, radioactive substances that can not be removed are accumulated on surfaces of water-contact portions of the components while the substances are recirculating in a recirculation system and so on together with the reactor water. As a result, radiations are radiated from the surface of the components, which becomes a cause of radiation exposure on person engaged with regular inspection during the inspection. A dose of radiation exposure under working is managed not to exceed a specified value for each person, however, recently the specified value is raised and there occurs the necessity that the exposure dose for each person is reduced as low as economically possible.
Therefore, various methods such as a method of reducing deposition of radionuclides on piping, a method of reducing the concentration of radionuclides in the reactor water, etc. are studied. For example, there is proposed a method of injecting metal ions such as zinc into the reactor water, forming a dense oxide film including zinc on the water-contact surface of piping of a recirculation system and suppressing to take radionuclides such as cobalt 60, cobalt 58, etc. into the oxide film (Patent literature 1). Further, there is proposed a method of forming a pre-oxidation film under a certain condition on inner surfaces of recirculation system piping and a reactor-water purifying system in which the reactor water flows during operation, before reaching to the condition that the radionuclides are solved out or released (Patent literature 2)
Patent literature 1: JP 58-79196 A
Patent literature 2: JP 62-95498 A